Method of improving the surface of galvanized steel material

ABSTRACT

THE PRESENT PROCESS FOR IMPROVING SURFACES OF GALVANIZED STEEL MATERIALS, ESPECIALLY THE PROPERTIES OF COATED SURFACES OF STRIP, BESIDES RENDERING THE SURFACES IN CONDITION FOR PAINT ADHENSION WITHOUT DETRACTING FROM THE FORMING PROPERTIES OF GALVANIZED STEL MATERIALS.

1973 TADAO KIMURO ETAL 3,712,826

METHOD OF IMPROVING THE SURFACE OF GALVANIZED STEEL MATERIAL Filed April 25. 1969 INVENTORY TADAO K/MURO AND ATTORNEYS United States Patent Int. Cl. B44d1/08, N06

US. Cl. 117-22 Claims ABSTRACT OF THE DISCLOSURE The present process for improving surfaces of galvanized steel materials, especially the properties of coated surfaces of strip, besides rendering the surfaces in condition for paint adhesion without detracting from the forming properties of galvanized steel materials.

DESCRIPTION The present invention is concerned with methods for improving the surfaces of galvanized steel materials.

It is Well known that spangle, frost flower coating, occurs on surfaces of galvanized steels, especially in the case of strip material, and it can be eliminated and flattened by the following methods. First, is to remove the spangle to make the material spangle free by heating the strip just after the coating to alloy the whole coated layer with iron-zinc in order to provide a smooth surface. Secondly, is a method which fines zinc crystals by spraying water or steam onto hot-dip galvanized surface. Third, is a method which fines zinc crystals by similarly blowing fine powder of metal oxide onto the hot-dip galvanized surface. Fourth, is a spangle minimizing method by roll quench.

However there still remains problems in those methods, that is, since in the galvannealing method the whole coated layer becomes an alloy of iron-zinc and it is hard and brittle, and forming the coatings cannot be avoided, especially this tendency is remarkable as the coated layer becomes thicker, and when the coating thickness goes beyond a certain extent, the coated steel cannot endure utilization. Furthermore it is difiicult to alloy uniformly along the entire area of the coated surface on the strip, and corrosion resistance is generally bad, besides if the fabricated portion is kept in the damping atmosphere, it has the disadvantage of creating reddish rust. There have been such defects as insufficient in fining crystals or lacking of improvement for paint adhesion in multiplication of crystalline nucleus owing to the methods of spraying water or steam, or rapidly cooling the material.

The present invention has solved the above mentioned prior defects or disadvantages, that is, there are no spangles and it further provides good paint adhesion, of course without detracting from the forming of the coatings. The basic object of this invention exists in getting the method to provide galvanized steel materials having surfaces which are non-spangled and with excellent paint adhesion without detracting from the formability of the galvanized layer. For this purpose, this invention sprays uniformly aqueous solutions of metal salts which can alloy with zinc onto galvanized surfaces at more than 420 C. so as to form the thin alloy layer of this metal and zinc on said coated surface, thereby bringing about no alterations in the thickness of the galvanized layer or adherence, besides obtaining superior qualities in paint adhesion by the thin alloy layer. Products made according to 3,712,826 Patented Jan. 23, 1973 this invention are fairly improved in corrosion resistance of the galvanized surface and high valued ones.

Another object of this invention is to obtain a method for applying to such products that have various sizes of spangle. For this purpose, the temperature of the strip on which said metal salts are sprayed should be chosen suitably in the range of more than 300 C. for this invention. The surface properties of the thus formed alloy layer can be suitably changed in accordance to purpose. Alloy surfaces having no spangle .is generally formed by spraying metal salt to the galvanized layer while coated zinc is still molten. On the other hand, when the above mentioned metal salt is once applied to the galvanized layer at 300 to 418 C, the formed spangle remains as it is, besides improvements in the corrosion resistance of said galvanized surface and paint adhesion can be achieved as mentioned above.

Further another object of the present invention is to accomplish economically and advantageously the improving purpose for galvanized layer having the above stated characteristics. For this purpose it is recommended to apply the present invention to the galvanized layer which has just been drawn up from the bath in accordance with the ordinary hot-dip galvanizing process. Such a method enables an apparatus to easily provide a structure of spraying said metal salt to accomplish economically and advantageously the desired purpose by utilizing residual heat after the hot dipping.

Many other characteristics or working effects will be clearly understood through the following explanation of the embodiments hereafter mentioned.

The accompanying drawing is a photomicrographic illustration showing the cross-section of a galvanized steel coating prepared by the embodiment of this invention.

To explain the embodiments of this invention with reference to the drawing, in the present invention powders or aqueous solution of metal salts are sprayed to alloy with zinc to said galvanized steel material at more than 300 C., which contains the following both cases, that is, spraying powders or aqueous solution to the coated surface which is still molten such as just after zinc coating on steel material; and spraying powders or aqueous solution to the coated surface whose steel material has been cooled or once cooled down to ordinary temperature, and again heated up to 418 to 300 C. This invention has such a characteristic that aqueous solutions or fine powders of halide, nitrate or sulfate of the metal are sprayed to alloy with zinc at such temperatures, thereby forming the thin alloy layer of said metal and zinc on the coated surface.

The process which is recommended for obtaining economical and useful products having no spangle is carried out in the following Way. Strip continuously immersed into the bath is taken out after having been adjusted to suitable coatings by means of the coating roll or gaswiping methods, and while coated zinc of this strip is still molten, powders of said metal salt or aqueous solutions are uniformly sprayed onto both. sides of the hot-dip galvanized surface. Then, the metal salts are decomposed to form the thin layer alloy on the galvanized surface.

Since this alloy layer is very thin, the formability of galvanized strip is not detracted at all. Spangling peculiar to the galvanized steel sheet fades away by the formation of this thin alloyed layer, bringing about uniform White grey appearance from one side edge of the strip to the other. Further, it is superior in corrosion resistance of the coating. To compare the galvannealed products with this present product concerning the strip of 2.3 mm. in thickness which has coatings of 20 microns in thick- 3 ness, the salt spray test based on 118 Z 2371 (Japanese Industrial Standard Z 2371) has proved that in the former reddish rust occurred after six hours while the latter created no reddish rust even after two hundred hours.

'In case intending to improve corrosion resistance of a galvanized surface as well as paint adhesion, leaving spangle as it is, powders or solutions of metal salts are sprayed to form the thin alloy layer under the condition that the strip cools a little after having been taken out of the bath and the galvanized layer is solid in the extent of 418 to 300 C. Note that the melting point of zinc is 419 C. When the temperature of coated material such as strip is under 300 C., it is diificult to form suitable alloy layer by decomposition of halide, nitrate or sulfate of metal to alloy with zinc.

The above mentioned is the statement of the economically useful method for production, but it is needless to say that the present method is not only limited to such a hot dipping method, but it is equally applicable for the electroplated zinc layer or another coated layer by other methods. If the one according to the hot-dip galvanized method, or which has been once cooled to the ordinary temperature or under 300 C. is again heated up to the above said temperatures and is appropriately molten. This invention can be just similarly applied to it.

Iron is most usually adopted as the metal which can be alloyed with zinc, but sometimes copper or tin can be also chosen. The present method uses as the surface treating agent one or more than two of a halide, nitrate or sulfate of those metals and a very thin film is formed on the zinc surface. This is the most important purpose of the invention.

In the case of directly spraying those salt powders to the galvanized layer, attention must be paid to, since the particle size of powder greatly effects to the properties of the treated surface. Generally, effective is the particle size of 30 to 150 mesh, especially 60 to 100 mesh. Powders which are coarse more than 30 mesh are difiicult to perfectly accomplish alloy with zinc uniformly, on the contrary, powders which are less than 150 mesh are too fine to contact thoroughly with the molten zinc surface, therefore almost all powders are oxidized before reacting on the surface zinc. At spraying aqueous solution, its concentration is suitably selected, but it is preferable to be as high as possible.

As mentioned above some salts are used as a surface treating agent. However the effect of surface-alloying may be more promoted by means of any one or a mixture selected from Group I or Group II of the Periodic Table, complex salts above mentioned salt and fluoride of boron titanium or silicon; sodium chloride, calcium chloride, ammonium chloride, ammonium fluoride, ammonium hydrogen fluoride, zinc chloride, sodium borofiuoride, ammonium siliconfluoride, sodium titanium fluoride, is further added to the spraying of powders or aqueous solution of the above metal salts as an accelerating agent of said reaction or that of the group consisting of the diluent zinc white, alumina or talc, as a diluting agent. In the case of a solution the evaporating hydrochloric acid or nitric acid is appropriately added to the solution to adjust the acidity, thereby effecting advantageously.

In this connection, these accelerators are not critical but assistant agents for promoting the alloying. The main agents is any one or a mixture selected from the group including halide, nitrate and sulfate of metal.

The galvanized strip through this invention method als brings the satisfactory results in the paint adhesion in each case of spraying metal salt powders and spraying aqueous solution, that is, the coated surface obtained by this method has the paint adhesion equal to that of products of galvannealing method in the case of using iron salt at spraying. When melamine paint was coated on the surface of galvanized strip obtained in the later mentioned Examples 3 and 4 so as to examine its adherence, it showed the excellent equal to that of the products of 4 the galvannealing method. However, in the case of the products according to the above mentioned other conventional spangle-minimizing methods, the satisfactory paint adhesion could not be obtained without the surface treatment for painting.

The above mentioned concerns the galvanizing of strip, and the present method is applied for not only strip, but also steel plate, steel tube, steel wire, and other general steel materials. The method of applying surface treating agents to the galvanized surface is not limited only to spraying, but adopts the roll application, electrostatic spraying or any methods available.

The present method brings galvanized steel products which have been voluntarily adjusted from the silver-grey and uniform surfaces having no spangle to the surface with minimized spangle having dots of alloy of zinc and said metal. Since the above said alloy layer is very thinly formed on the coated surface, the forming properties of the coating is kept well which is the same as the conventional hot-dip coating. The treating method thereof is enough to spray metal salt to the surface to alloy for short period. The surface after treating is often partially covered with black-brown oxides which are easily removed by washing with brushing.

Several examples of this invention are as follows:

Example 1 When the powders of ferrous chloride of 60 mesh were sprayed 1 g./m. to the surface of galvanized steel strip at a temperature of 450 C. such a surface was obtained which had minimized spangle with rough surface of ironzinc alloy.

Example 2 When the particle size of ferrous chloride of Example 1 was changed into mesh, such a surface was obtained as approximately uniform iron-zinc alloy.

Example 3 When 30% aqueous solution or ferrous chloride was sprayed 50 mL/m. to the surface of galvanized steel strip at a temperature of 450 C., such analloyed surface was obtained as silver-white, uniform, smooth, 2 to 3 microns in thickness and having no spangle. The photomicrograph of the cross-section of this strip surface has been illustrated in the drawing, wherein A is the steel substance, B is the zinc layer and C is the alloy surface according to this present treatment.

Example 4 When concentrated hydrochloric acid was added 20 ml./l. to the 50% aqueous solution of ferric chloride and it was sprayed 200 mL/m. to the galvanized steel strip at a temperature of 480 C., the silver-grey surface having no spangle was obtained.

Example 5 When ammonium chloride was added 30 g./l. to the 10% aqueous solution of ferrous chloride and it was sprayed ml./m. to the galvanized steel strip at a temperature of 430 C., the silver-white surface of thin alloy layer was obtained which had no spangle at all.

Example 6 When the 50% aqueous solution of ferrous chloride was sprayed 30 ml./m. to the solidified surface of the galvanized layer which was cooled down to 380 C., the silver-grey surface of thin iron-zinc alloy layer was obtained on the uneven zinc surface having spangle.

Example 7 When the 30% aqueous solution of ferrous nitrate was sprayed 100 mL/m. to the surface of the galvanized steel strip at a temperature of 450 C. the silver-grey surface having no spangle was obtained.

Example 8 When the solution containing ammonium ferrous sulfate and 1% ammonium silocofiuoride was sprayed 100 mL/m. to the surface of the galvanized steel strip at a temperature of 450 C., the thin silver-grey surface having no spangle was obtained.

These solutions were sprayed to ml./m. to the galvanized steel strip at a temperature of 450 C., which was still molten just after zinc coating. These products showed excellent paint adhesion, forming properties and corrosion resistance.

We claim:

1. Method for improving the surface of galvanized steel consisting in zinc-coating a steel by a hot-dip galvanizing process, spraying powders or aqueous solution of one or a mixture selected from the group including halide,

nitrate and sulfate of iron on said zinc-coating layer, while 30 still in its molten state and then further forming a thin alloying layer of said metal-zinc on said zinc-coating layer.

2. A method for improving the surface of galvanized steel consisting in zinc-coating a. steel by a hot-dip galvanizing process, spraying powders of 30 to 150 mesh, of one or a mixture selected from the group including halide, nitrate and sulfate of metal on said zinc-coating layer, while still in its molten state, and then further forming a thin alloying layer of said metal-zinc on said zinc-coating layer.

3. Method of improving the surface of galvanized steel material as described in claim 1 including adding sodium borofluoride as reacting accelerator.

4. Method of improving the surface of galvanized steel material as described in claim 1, including using powders of to mesh as powder composed of sole or mixture of halide, nitrate or sulfate of metal to alloy with 21110.

5. Method of improving the surface of galvanized steel as described in claim 1, including using as reacting accelerator wherein a mixture of the Groups A, B and C are further added as a reacting accelerator,

Group A: chloride or fluoride of Li, K, Na, NH,

Group B: chloride of Mg, Ca, Zn, Cd, Ba

Group C: Na, K, NH complex salts of B134, SiF

TiF

References Cited UNITED STATES PATENTS 2,126,244 8/1938 Cook ct a1 29--l96.5 3,379,557 4/1968 Hoover et al ll7114 X 3,323,938 6/1967 Vaught 1l7130 R EDWARD G. WHITBY, Primary Examiner U.S. Cl. X.R.

29l96.5; l1764 R, 71 M, 105, 114 A, 131 

